1. Technical Field
This disclosure relates to DC/DC converters and to saturation of transformers used therein.
2. Description of Related Art
DC/DC converters are widely used to step from an input voltage to an output voltage that may be higher or lower than the input voltage. Some DC/DC converters employ a transformer to step the input voltage up or down and/or provide galvanic isolation between the input supply and the output supply (i.e. isolated converters).
When a voltage is applied to a primary transformer winding, the magnetic field in the transformer core creates a voltage on the secondary transformer winding that is proportional to the turns ratio of the transformer. The turns ratio is the ratio of the number of turns on the primary winding (NP) to the number of turns on the secondary winding (NS). The voltage on the secondary side of the transformer is therefore VSEC=VPRI*NS/NP. The relationship between the current that is demanded by the secondary side and the current supplied by the primary side of the transformer is inversely proportional to the turns ratio, such that ISEC=IPRI*NP/NS.
An additional “magnetizing current” is required to magnetize the core of the transformer. This magnetizing current flows on the primary side of the transformer, but not on the secondary side of the transformer.
The magnetizing current relationship to voltage is inductive. In other words, when voltage is applied to the primary winding of the transformer, the magnetizing current increases linearly as the magnetic flux density (“flux density”) accumulates inside the core. With continued positive bias across the primary winding, the transformer will saturate, causing the transformer windings to appear as a short circuit. At this time, the magnetizing current increases very rapidly and is limited only by a small amount of leakage inductance, which is generally 100-1000 times less than the magnetizing inductance. This rapid increase in current can be catastrophic for a DC/DC converter and may destroy components.
To avoid saturation, the flux density in a DC/DC converter can be reduced through an operation referred to as “resetting” the core. These DC/DC converters typically cycle between a power transfer phase during which power is transferred from the input to the output of the converter and a reset phase during which the flux density that increased during the power transfer phase is reduced.
Reducing the flux density is typically accomplished by reversing the voltage on the primary winding. Some DC/DC converters use what is known as an “active clamp reset.” The reversing voltage is applied until the flux density not only drops to zero, but then increases in the opposite direction. If allowed to increase too much in the opposite direction, however, the core of the transformer can also saturate during the reset phase.
Various approaches have been taken to switch between the power transfer phase and the reset phase without causing saturation.
Some converters use a primary side current comparator. These converters compare the transformer current on the primary side during the power transfer phase with a threshold. When the threshold is exceeded, they shut off a primary side switch that controllably connects the input of the converter to a supply source. However, the comparators that are used may not be fast enough to protect the switch or other components from damage in the event of the rapid increase in current due to transformer saturation. Also, these types of converters do not protect the circuit from saturation during the reset phase.
Some converters slow down the rate at which increases in current are permitted, making fast detection of saturation less important. However, this slow down reduces the ability of the converter to quickly compensate for rapid changes in the source or load, weakening its ability to provide tight regulation. This approach may also fail to avoid saturation, such as when the input voltage rapidly ramps to its minimum value.
Some converters limit the maximum duty cycle of the power switch that controls the length of time the input of the converter is connect to the supply source during the power transfer phase, as transformer saturation is often a greater problem at high duty cycles, when the on-time is long. This approach, however, does not prevent negative flux saturation during the reset phase. It may also fail to prevent transformer saturation during lower duty cycles of the power transfer phase, such as when the converter is started up with its output initially pre-biased at its normal operating level.